Thermoluminescent aerosol analysis

ABSTRACT

A method for detecting and measuring trace amounts of aerosols when reacted with ozone in a gaseous environment wherein a sample aerosol is exposed to a fixed ozone concentration for a fixed period of time, a fluorescer added to the exposed sample and thereafter the sample heated in a 30° C/minute linear temperature profile to 200° C. undergoes thermoluminescence the trace peak thereof is measured and recorded as a function of the test aerosol and wherein the recorded thermoluminescence trace peak of the fluorescer is specific to the aerosol being tested.

ORIGIN OF THE DISCLOSURE

The invention described herein was made by employees of the UnitedStates Government and may be manufactured and used by or for theGovernment for governmental purposes without the payment of anyroyalties thereon or therefor.

BACKGROUND OF THE INVENTION

The present invention relates to a method for detecting and measuringtrace amounts of various aerosols in a gaseous environment. As usedherein the term aerosol relates to the organic and inorganic pollutantmaterials, usually solids, that are suspended within the earth'satmosphere. In one aspect, the invention relates to a method fordetecting and measuring specific aerosols which could aid in thedevelopment of a detector for rapid identification of effluent sources.Also, the invention could serve as an experimental tool for study ofheterogeneous chemistry and study of formation of toxic, electronicallyexcited species in airborne particles.

There are many known methods of determining the presence of traceimpurities or various pollutants in a gas such as the earth'satmosphere. However, several of these methods require the use of devicesthat are cumbersome, expensive, or both, and there is a need for asimple and inexpensive reliable technique. This need is particularlyacute in spacecraft and other installations where weight and bulkinessare of primary importance. Previous methods for aerosol analysis includegas and liquid chromatography, mass spectroscopy, electron microscopy,x-ray fluorescence, and wet chemical analysis. Methods for measuringaerosol concentration include high volume samples, turbidity meters, andother particle counting instruments, none of which measure chemicalcomposition.

A previous process for measuring trace amounts of ozone, nitrogen oxideand carbon monoxide, and similar in some respects to the presentinvention is disclosed in U.S. Pat. No. 3,977,831. This patented processinvolved pollution detection wherein the pollutant reacted with a solidorganic material that inherently chemiluminescensed when heated and thetotal integrated light intensity, measured during the heated cycle beinga measure of pollutant exposure. In the present invention an aerosol isreacted with an ozone environment and the reactant product exposed to afluorescer. The sample is then subjected to a heating profile, linearlyprogrammed at 30° C. per minute to a temperature of 200° C. The peakintensity of the fluorescer thermoluminescence during this heatingserves as an indication of the aerosol tested.

Accordingly, it is an object of the present invention to provide asimple and reliable technique for detecting and measuring trace amountsof various aerosols in a gas. It is further an object to provide such aprocess which utilizes organic materials that undergo chemical changesand serve as transfer agents or indicators of selected aerosol/ozonereactions and thereafter undergo chemiluminescence when heated.

Another object of the present invention is to provide a process ofmeasuring peak intensity of a thermoluminescent reaction as a functionof a specific aerosol.

BRIEF SUMMARY OF THE INVENTION

According to the present invention the foregoing and other objects areattainable by exposing a measured sample of a known aerosol to an ozonegaseous environment, adding a fluorescer to the exposed sample andheating the combined materials at a linear rate of 30° C/minute to atemperature of 200° C. The peak light intensity measured during theheating cycle is a measure of the specific aerosol being tested.

The organic material suitable for detecting trace quantities of aerosolsaccording to the present invention is selected from the group consistingof rubrene, napthacene, poly(ethylene 2, 6-naphtahalene dicarboxylate),9, 10 dibromo anthracene and 9, 10 diphenyl anthracene. These materialsare available as dry solids and are dissolved in a suitable solvent foruse in the present invention and must have the inherent chemicalproperty characteristics of being excited by energy transfer from theozonide in the aerosol.

DETAILED DESCRIPTION

A more complete appreciation of the invention and many of the inherentadvantages thereof will be more clearly understood by reference to thefollowing detailed description when considered in connection with thespecific examples and accompanying drawings wherein:

FIG. 1 is a schematic diagram of an aparatus useful for measuring thepeak thermoluminescent intensity from the aerosol/ozone reactionaccording to the present invention, and

FIG. 2 illustrates the recorder trace from a specific aerosol measuredaccording to the present invention.

Referring now to the drawings and more particularly to FIG. 1, there isshown the apparatus generally designed by reference numeral 10 fordetecting and measring peak thermoluminescence according to the presentinvention. Apparatus 10 includes a housing 12 containing a conventionalphotomultiplier tube 14 having yellow response and in electricalconnection with a suitable power supply 16. A sample container 18 thathouses the aerosol that is to be measured for thermoluminescence ispositioned in sample holder 20 adjacent programmed heater element 21.Electric leads 22, 23 connect heater element 21 with a suitable powersupply and controls, not shown, that increases the heater output toinsure a sample increase of 30° C. per minute up to a temperature of200° C. A thermocouple 24 is in electrical connection with one of thepens on dual pen strip recorder 26 via lead wire 28. The other pen ofrecorder 26 is in electrical connection with an electrometer 30 via lead31 to record the output received through lead 32 from photomultiplier14.

Housing 12 is provided with a pair of openings in the sidewall thereofas designated by reference numerals 34 and 36 shown closed,respectively, by covers 35 and 37. The various components of apparatus10 are conventional, commercially available items, for example,photomultiplier 14 may be an RCA 7265 photomultiplier; electrometer 30an Elcor (Model A309B) electrometer available from Elcor, Inc., FallsChurch, Virginia, and dual pen recorder 26 is an Electronik 194 fromHoneywell, Inc., Fort Washington, Pa. 19034. Thermocouple 24 is aconventional chromel/alumel thermocouple soldered or otherwise thermallyconnected to sample holder 20 and electrically monitored by recorder 26to continuously record the temperature of sample 18 as heated. Thetemperature of heater element 21 was controlled by F&M ScientificCorporation, Model 240M temperature programmer (not shown). When testingfor aerosol/ozone reactions according to the present invention, theaerosol sample employed is in solid form. Samples of these aerosols maybe collected on sterile filters or like surfaces that are not reactiveto the ozone in the environment. An example surface suitable forcollection of these samples is a glass surface that has been bleachedout relative to ozone reaction.

The operation of the device described above is now believed apparent. Inone application of the invention, the sample container 18 housing 2 mgof the sample to be tested for thermoluminescence is placed in sampleholder 20 through one of the openings 34, 36. A mixture of 1% ozone inoxygen is then pumped into evacuated housing 12 to provide the gaseousenvironment therein and the container covers 35 and 37 replaced to closethe container.

In this test the samples were exposed to the ozone and oxygenenvironment for approximately five minutes and covers 35 and 37 thenremoved to permit ambient air to enter housing 12. After 5--10 minutes,allowed for excess ozone to dissipate, a drop of saturated solution ofrubrene in benzene was added to the sample in container 18. The benzeneevaporates rapidly, usually 2-5 minutes, and the remaining rubrene isdispersed throughout the aerosol/ozone solid and serves as thefluorescent material which luminesced when excited by the thermaldecomposition of the ozonides. This sample is then heated by programmedheater element 21 at a linear rate of 30°/min to a temperature of 200°C. The light emitted by thermoluminescence, is detected byphotomultiplier 14, amplified by electrometer 30 and recordedsimultaneously with the temperature on dual pen recorder 26.

The light output from the sample rises to a maximum as shown in thegraph of FIG. 2 for a sample of 1,2 benzanthracene and then decays tozero as the temperature approaches 200° C.

The peak light intensity measured during the heating cycle is a measureof the aerosol being tested. Thus, the luminescent curve obtained has aline shape and maximum that is characteristic to the specific aerosoltested. The parameters listed in the Table below were measured from therecorded output for each of the substances tested.

                  TABLE                                                           ______________________________________                                        OZONE/AEROSOL THERMOLUMINESCENCE                                                                               Maximum                                                   Initial   Initial   Four Hr After                                Substance    Maximum   Halfwidth Ozonation                                    ______________________________________                                        1293 Aerosol 97° C.                                                                           81° C.                                                                           100,122,151° C.                       Rogo Aerosol  83       64        102                                          Ammonium Sulfate                                                                           110       44        --                                           Sodium Chloride                                                                            152       52        --                                           3,4 Benzpyrene                                                                             103       69        --                                           1,2 Benzanthracene                                                                          99       35        --                                           Coronene      85       72        101                                          N-Octacosane 122       96        130                                          Al.sub.2 O.sub.3                                                                           No reaction                                                      ______________________________________                                    

A comparison of the parameters in the Table indicates that the varioussubstances are distinguishable on the basis of this simple analysis ofthe glow curves. The temperature maxima or peak intensity and thelinewidths at half maximum are all different. The organic compounds inthe Table are known constituents of urban aerosols. The cosanesconstitute 85% of the organics in urban aerosols. Ammonium sulfate is aprimary sulfate in the atmosphere and a product of SO₂ oxidation. NaC1is an aerosl in the coastal regions. Aluminum oxide did not display areaction with ozone. A more detailed analysis of line shapes shouldreveal other characteristics that are unique to the substance tested.Only the line shape for 1,2, benzanthracene (FIG. 2) is included in thepresent application in the interest of clarity. The other line shapesare similar but with the noted different initial maximum and initialhalf width maximums.

The aerosols identified as "1293 aerosol" and "ROGO aerosol" werecollected at two different sites as precipitated particles from thelocal atmosphere and are mixtures of many compounds. The site selectionfor these two aerosols were not sterilized or in any manner prepared forthe sample collection. The glow curves obtained from these aerosols arereproducible and therefore could be compared with aerosls from knownpollution sources to identify the source. Such comparison would be aninvaluable aid for tracing dispersion of pollutants as well asdetermining the identity of the source.

The thermoluminescence curves may also be useful in qualitative analysisof the aerosol composition. This is accomplished by comparison of theaerosol luminescence curves with curves obtained from a mixture ofchemical species that are known constituents of aerosol. For a specificgeographical area there would be a limited number of species. A catalogof glow curves could be produced by measuring aerosols from varioussources and analyzing their chemical compostion by standard methods.Identification of the aerosol would then be accomplished by simplycomparing the flow curves obtained in the field to the cataloged curves.Also this catalog of curves could be stored in a computer and duringprintout of the "unknown" trace the corresponding curve peaks could becompared and the "unknown" readily identified.

Four of the samples in the Table above were examined four hours afterozonation and found to reproduce a glow curve different from those foundimmediately after ozonation. In the case of "1293 Aerosol", severalpeaks were exhibited. Since the ozonides formed initially are unstable,new chemical species are formed at various rates and glow curves such asthese provide additional data for characterizing and identifying theaersol constituents.

The above detailed description of specific tests is exemplary of thepresent invention and are by no means considered exhaustive. Theseexperiments were all ozonated in the laboratory at 1% ozoneconcentration. Interactions of (NH₄) HSO₄ with ozone at ozoneconcentrations of one part per million (ppm) have also been note. Traceamounts of ozone of this concentration are present on the earth'satmosphere at all levels and it can therefore be assumed that airborneaerosols are ozonated naturally, although not to the extent producedunder the controlled laboratory tests as shown in the Table. However,with more sensitive photon detection devices, the ozonides producednaturally could be detected using procedures analogous to thesedescribed herein and the need for laboratory ozonation would beeliminated.

The temperature at which the peak intensity in the flow curve occurs isrepresentative of the stability of the ozonides formed and can berelated to activation energy for the thermal decomposition thereof. Fora reaction following first order kinetics, the position of the peakdepends only on the type of compounds formed and time after ozonation,and not in the concentration of these compounds in the mixture. Undersuch conditions the peak in the glow curve will not depend on the amountof ozone that has reacted with the aerosol and is therefore specific tothe aerosol.

It is thus seen that the present invention provides a process forreadily determining the presence and identity of various aerosols in thetest sample and can serve as an aid in determining the source of suchaersols in a specific geographic area. This is accomplished by theheterogeneous interaction occuring between the known various organic andinorganic atmosphere aerosols and ozone. This interaction is observablein the laboratory by utilizing a fluorescer as a transfer agent orindicator for the aerosol/ozone interaction and by using athermoluminescence technique to obtain a luminescent spectrum. Thisspectrum is different for each compound and is thus characteristic ofthe molecular structure of the compound. This spectra also changes withtime after ozonation and thus additional chemical reactions take placeafter the initial heterogeneous complexing. With available meterologicaldata at the test site, these properties make the present invention of asensitive thermoluminescent process applicable for detecting andtracking aerosols to their source, i.e., smoke stacks, chemical plants,steel mills, etc. Additionally, the instrumentation requirements for thepresent invention make the process readily suitable for sourceidentification in the field as well as for ground truth and flightexperiments.

Also, the present invention provides a useful tool for parameterevaluation in atmospheric chemistry models which include aerosols and inthe basic study of heterogeneous chemistry. The observations ofluminescence indicate that the aerosol/ozone complex is a high energyspecies with serious implications with respect to toxic biologicalproperties such as mutagenicity and carcinogenicity. Another uniqueadvantage of the present invention is the sensitivity of the process.Solid reaction rates as low as 10⁻⁹ mole/year and liquid reaction ratesas low as 10⁻¹⁴ mole/year can be measured by the process describedherein.

Although the invention has been described relative to use of preciseequipment and with specific illustrative examples, it is not so limited.There are obviously many modifications and variations of the presentinvention that will be readily apparent to those skilled in the art inthe light of the above teachings. It is therefore to be understood thatthe invention may be practiced otherwise than as specifically describedherein within the scope of the appended claims.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A method for detecting and identifying traceamounts of aerosols in an atmospheric environment comprising:collectinga sample of the aerosol to be tested; exposing the collected sample to acontrolled gaseous environment having a known quantity of ozone therein,for a fixed period of time, to thereby facilitate an aerosol/ozonereaction; adding a fluorescer indicator to the reacted sample; heatingthe combined sample at a linear programmed rate of 30° C/min to atemperature of 200° C.; detecting the thermoluminescence of thefluorescer during the heating cycle, and recording a trace of thethermoluminescent output during the heating cycle with the peak outputthereof being indicative of the specific aerosol.
 2. The method of claim1 wherein the aerosol tested includes a mixture of pollutants and therecorded trace includes specific peaks at specific temperatures in theheating cycle that are indicative of each individual pollutant.
 3. Themethod of claim 1 wherein the fluorescer is selected from the groupconsisting of rubrene, napthacene, 9,10 diphenyl anthracene and 9,10dibromo anthracene.
 4. The method of claim 1 including the step ofcomparing the recorded trace peak with known or suspected aerosolsources in a specific geographic area to aid in identifying thepollutant source.
 5. The method of claim 1 wherein 2 mg of the collectedsample is exposed to a controlled gaseous environment consisting of 1%ozone in oxygen for a period of five minutes.
 6. The method of claim 1wherein the fluorescer is dissolved in a solvent and added to theaerosol/ozone reaction product as a liquid.
 7. The method of claim 6wherein the solvent is benezene.